1. Field of the Invention
The present invention relates to an active matrix display apparatus using a light emitting element for a pixel and a drive method therefor. The invention also relates to an electronic device provided with such a display apparatus.
2. Description of the Related Art
In recent years, a flat panel light emitting display apparatus using an organic EL device as a light emitting element has been developed. The organic EL device is a device utilizing the phenomenon that light is emitted when an organic thin film is applied with an electric field. The organic EL device is driven at an applied voltage of 10 V or smaller and thus consumes a small amount of electric power. Also, the organic EL device is a light emitting element which emits light from itself. Therefore, the organic EL device does not need an illumination member, and accordingly, it is easy to realize a lighter weight and a thinner structure. Furthermore, the response speed of the organic EL device is about several μs, which is extremely fast, and therefore an after image during video display is not generated.
Among the flat panel light emitting display apparatuses using the organic EL device for the pixel, an active matrix display apparatus in which thin film transistors are formed as drive elements in each pixel in an integrated manner has been particularly developed. Such an active matrix, flat panel light emitting display apparatus is described in, for example, Japanese Unexamined Patent Application Publication Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, 2004-093682, and, 2006-215213.
FIG. 23 is a schematic circuit diagram of an example of an active matrix display apparatus in related art. The display apparatus is composed of a pixel array section 1 and a peripheral drive section. The drive section is provided with a horizontal selector 3 and a write scanner 4. The pixel array section 1 is provided with signal lines SL arranged in columns and scanning lines WS arranged in rows. Pixels 2 are arranged at positions where the respective signal lines SL intersect with the scanning lines WS. For facilitating an understanding, in the drawing, only one pixel 2 is illustrated. The write scanner 4 is provided with a shift register. The write scanner 4 is operated in accordance with a clock signal ck supplied from the outside, and sequentially outputs control signals to the scanning lines WS by sequentially transferring a start pulse sp similarly supplied from the outside. The horizontal selector 3 is adapted to supply the signal lines SL with video signals in accordance with the line progressive scanning on the write scanner 4 side.
The pixel 2 is composed of a sampling transistor T1, a drive transistor T2, a holding capacitance C1, and a light emitting element EL. The drive transistor T2 is of a P channel type. A source of the drive transistor T2 is connected to the power source line, and a drain of the drive transistor T2 is connected to the light emitting element EL. A gate of the drive transistor T2 is connected to the signal line SL via the sampling transistor T1. The sampling transistor T1 is put into a continuity state in accordance with the control signal supplied from the write scanner 4, and samples the video signal supplied from the signal line SL to write the video signal in the holding capacitance C1. The drive transistor T2 receives the video signal written in the holding capacitance C1 as a gate voltage Vgs at the gate, and allows a drain current Ids to flow into the light emitting element EL. As a result, the light emitting element EL emits light in accordance with the video signal. The gate voltage Vgs represents a potential at the gate while the source is used as a reference.
The drive transistor T2 is operated in a saturated area, and the relation between the gate voltage Vgs and the drain current Ids is represented by the following characteristic expression;Ids=(½)μ(W/L)Cox(Vgs−Vth)2 
where μ represents the mobility of the drive transistor, W represents the channel width of the drive transistor, L represents the channel length of the drive transistor, Cox represents the gate insulation film capacity in unit areas of the drive transistor, and Vth represents the threshold voltage of the drive transistor. As is apparent from this characteristic expression, when the drive transistor T2 is operated in the saturated area, the drive transistor T2 functions as a constant current source for supplying the drain current Ids in accordance with the gate voltage Vgs.
FIG. 24 is a graphic representation of a voltage/current characteristic of the light emitting element EL. The horizontal axis represents an anode voltage V, and the vertical axis represents the drive current Ids. It should be noted that the anode voltage at the light emitting element EL becomes a drain voltage at the drive transistor T2. In the light emitting element EL, as the current/voltage characteristic changes over time, the characteristic curve has a tendency to be flattening together with the elapse of time. For this reason, even when the drive current Ids is constant, the anode voltage (drain voltage) V changes. In that point, with the pixel circuit 2 illustrated in FIG. 23, the drive transistor T2 is operated in the saturated area, and irrespective of the variation of the drain voltage, the drive current Ids can be flown at the gate in accordance with the voltage Vgs can be flown at the gate. Therefore, irrespective of the characteristic change over time of the light emitting element EL, it is possible to maintain the light emission luminance constant.
FIG. 25 is a circuit diagram of another example of the pixel circuit in related art. A difference from the pixel circuit previously illustrated in FIG. 23 is that the drive transistor T2 is of an N channel type instead of the P channel type. For a manufacturing process of the circuit, it is advantageous in many cases to set all the transistors configuring the pixels to the N channel type.